From the reference site you provided (using the compare function), the GTX 1080 Max-Q is roughly 14,5% slower than a GTX 1080 desktop; roughly 5,6% slower than a desktop GTX 1070; roughly 5,7% faster than a desktop GTX 1060 (the extra RAM also doesn’t hurt in the GTX 1080 Max-Q). I’ve read other sources stating that the GTX 1080 Max-Q can be up to 25% slower than the desktop GTX 1080. Performance wise, being somewhere between a desktop GTX 1070 and desktop GTX 1060 isn’t a bad spot to be.
A GTX 1070 Max-Q is roughly equivalent to a desktop GTX 1060 (not 3GB or 5GB versions).
You do get laptops / notebooks with desktop equivalents of the GTX 1080 / 1070 / 1060, but that too is a compromise as they’re not thin and light (or inexpensive), in fact you can get a laptop with 2x desktop grade GTX 1080’s but I’ll admit they’re not for everyone.